Micturition is thus characterized by:
- relaxation of the striated sphincter (somatic innervation)
- relaxation of the smooth muscle sphincter and opening of the bladder neck (sympathetic innervation)
- detrusor contraction (parasympathetic innervation)
What is the micturition reflex?
The micturition reflex involves a coordinated and sustained contraction of the detrusor muscle (the detrusor reflex) along with simultaneous relaxation of the urethra. Detrusor contraction without urethral relaxation is called reflex dyssynergia. One may also ask, what is the process of micturition?
What is bladder emptying and micturition reflex?
Bladder Emptying and the Micturition Reflex. The micturition or emptying phase displays a coordinated relaxation of the inner and outer urethral sphincters, under sympathetic and somatic regulation respectively, with strong contractions of the detrusor muscle due to parasympathetic impulses.
What is the volume of urine that initiates the micturition reflex?
Stimulus: Volume of urine that initiates micturition reflex is 300-400 ml 2. Receptor: Stretch receptors in bladder wall 3. Afferent: Pelvic parasympathetic
What muscles are involved in micturition?
The abdominal wall and pelvic floor musculature also participates by increasing the force on the bladder to help achieve complete emptying. The act of micturition is an autonomic reflex at the level of the spinal cord.
What is the act of micturition?
The act of micturition is an autonomic reflex at the level of the spinal cord. This reflex also helps to complete micturition when the act is voluntarily initiated, or when it follows a period of inhibition by the brain, by relaxing the external sphincter. The control of this process is mediated via afferent signals from stretch ...
What is the guarding reflex?
Urethral reflexes, called ‘the guarding reflex,’ also play a part in inhibiting involuntary bladder emptying during this process. The afferents are all conveyed through the pelvic nerves to initiate a spinal reflex.
What is the role of the cell group in the periaqueductal gray (PAG) in detecting
This includes social, sensory, and emotional states, including the degree to which bladder stretching is sensed to be safe and tolerable. The cell group in the periaqueductal gray (PAG) plays a role in detecting the bladder distension , as well as in relaying bladder afferents to higher centers in the brain and enabling the person to feel the sensation. It also regulates the feed to the pontine center, while receiving afferents from higher brain centers such as the anterior cingulate and the prefrontal cortex. These help to inhibit the voiding reflex via suppression of PMC excitation.
What is the pontine micturition center?
Pontine Micturition Center. The pontine micturition center (PMC) in the brainstem is activated via afferent signals from the urinary bladder as it is filling. This center sends inhibitory impulses to the spinal reflex arcs to enable bladder voiding. In the absence of any other regulation, the afferents from the bladder and urethra to ...
What is the detrusor in the bladder?
The detrusor is the smooth or involuntary muscle of the bladder wall. The urethral muscles consist of the external and internal sphincter. The internal sphincter and detrusor muscle are both under autonomic control. The external sphincter, however, is a voluntary muscle under the control of voluntary nerves.
What is the term for the relaxation of the striated sphincter?
Micturition is thus characterized by: relaxation of the striated sphincter (somatic innervation) relaxation of the smooth muscle sphincter and opening of the bladder neck (sympathetic innervation) detrusor contraction (parasympathetic innervation) The distension of the urinary bladder wall causes wall tension to rise very slightly.
What is the emptying phase of the urethral sphincter?
The micturition or emptying phase displays a coordinated relaxation of the inner and outer urethral sphincters, under sympathetic and somatic regulation respectively, with strong contractions of the detrusor muscle due to parasympathetic impulses.#N#Micturition is thus characterized by:
What is the micturition reflex?
The micturition reflex is peripherally mediated by components of the somatic and the autonomic nervous systems. The bladder receives its motor innervation through the parasympathetic pelvic nerves. The principal nuclei innervating the urinary bladder of the cat consist of motor neurons located in the gray matter of the intermediolateral cell column of the sacral spinal cord and motor neurons in the ventral gray matter of the sacral spinal cord in the region of Onuf's nucleus. In humans, sacral nerve blocks have revealed that the detrusor nucleus has a rostral–caudal extension going from the S3 to S4 segment. The precise intramedullary location of detrusor motor neurons and their histological characteristics in the sacral spinal cord have not been described.
Which nerve is the target of the micturition reflex?
Many of the sensory afferent nerve fibers contained in the sacral spinal nerves originate in the pudendal nerve, thereby making the pudendal nerve an ideal target for neuromodulating inhibition of the micturition reflex.
How does sacral neuromodulation work?
Sacral neuromodulation is thought to improve bladder storage by inhibiting the micturition reflex via electrical stimulation of sensory afferent fibers , in particular by depolarization of Aα and Aγ somatomotor fibers that affect the pelvic floor and external sphincter and thus inhibit detrusor activity (Hohenfellner et al., 1992, 1998). Many of the sensory afferent nerve fibers contained in the sacral spinal nerves originate in the pudendal nerve, thereby making the pudendal nerve an ideal target for neuromodulating inhibition of the micturition reflex. Direct pudendal nerve neuromodulation stimulates more pudendal afferents than SNS and may do so without the side effects of off-target stimulation of leg and buttock muscles. Thus, techniques for direct pudendal nerve stimulation at alternative locations to the sacral foramen are being developed.
What are bladder reflexes?
These interneurons synapse with preganglionic efferent parasympathetic nerves to complete this reflex pathway. The bladder-bladder reflex is an excitatory one, which becomes activated by the sensing of a full bladder but is inhibited until it is socially appropriate to void. Interneurons activated by bladder afferent fibers also synapse with urethral parasympathetic efferent nerves to form a “bladder-urethral reflex” (Leng and Chancellor, 2005). This is an inhibitory reflex, which relaxes the smooth muscle of the proximal urethra and bladder outlet to open immediately before the onset of bladder contraction.
What is the term for a detrusor contraction without urethral relaxation?
Detrusor contraction without urethral relaxation is called reflex dyssynergia. 31.
When did the first evidence for sacral micturition reflexes come out?
The first evidence for the existence of sacral micturition reflexes was given by De Groat (1975) and De Groat et al., (1981 ), who observed that micturition as well as defecation are elicited in neonatal kittens when the mother licks the perineal region.
Which neuron receives input from the cerebral cortex, cerebellum, basal ganglia, thal
Micturition depends on a spinobulbospinal reflex that is relayed through the pontine micturition center, which receives input from the cerebral cortex, cerebellum, basal ganglia, thalamus, and hypothalamus, and is the final common pathway to the bladder motor neurons.
How is micturition mediated?
Micturition is mediated by activation of the sacral parasympathetic efferent pathway to the bladder and the urethra (Figure 76.6b) as well as reciprocal inhibition of the somatic pathway to the urethral sphincter ( Table 76.2) ( Figure 76.6b ). Studies in animals using brain lesioning techniques revealed that neurons in the brain stem at the level of the inferior colliculus (i.e. the PMC) have an essential role in the control of the parasympathetic component of micturition ( Torrens and Morrison, 1986; Mallory et al., 1991; de Groat et al., 1993 ). Removal of areas of the brain above the inferior colliculus by intercollicular decerebration usually facilitates micturition by elimination of inhibitory inputs from more rostral centers ( Yokoyama et al., 2000 ). However, transections at any point below the colliculi abolish micturition. Bilateral lesions in the rostral pons in the region of the locus coeruleus in cats or Barrington’s nucleus in rats also abolish micturition, whereas electrical or chemical stimulation at these sites triggers bladder contractions and micturition ( Mallory et al., 1991 ). These observations led to the concept of a spinobulbospinal micturition reflex pathway that passes through the pontine micturition center (PMC) ( Figure 76.6b ). The pathway functions as an “on–off” switch that is activated by a critical level of afferent activity arising from tension receptors in the bladder and is in turn modulated by inhibitory and excitatory influences from areas of the brain rostral to the pons (e.g., diencephalon and cerebral cortex) ( Figure 76.7) ( Torrens and Morrison, 1986; de Groat et al., 1993 ).
How does micturition occur in animals?
In animals with an intact spinal cord, micturition is initiated by a supraspinal reflex pathway passing through a center in the brain stem. The pathway is triggered by myelinated afferents (Aδ-fibers), which are connected to the tension receptors in the bladder wall.
What is the purpose of a detrusor contraction?
A normal detrusor contraction is of adequate strength and duration to empty the bladder completely in one coordinated contraction.
What are the phases of micturition?
The micturition cycle involves two phases: bladder filling/urine storage and bladder emptying. Bladder filling requires (i) accommodation of increasing volumes at low intravesical pressure (compliance) and appropriate sensation, (ii) the bladder outlet to be closed at rest and during increases in intra-abdominal pressure, and (iii) the absence of involuntary bladder contractions. Bladder emptying requires (i) coordinated contraction of the bladder of adequate magnitude and duration, (ii) lowering of resistance at sphincters, and (iii) the absence of obstruction (e.g., enlargement of the prostate gland in the aging male may result in bladder outlet obstruction, precluding efficient micturition). Bladder dysfunction can then be clinically identified as a problem of filling or emptying or a combination of both, and the site of dysfunction may be the bladder, the urethra, or both.
Why do fast twitch fibers contract?
With rapid increases in intra-abdominal pressure, fast twitch fibers are recruited to contract and further increase the urethral resistance to avoid urinary leakage. As the bladder fills, its visceral afferents travel through the peripheral nerves, ascending through the spinal cord to the pontine micturition center.
Which nerves are involved in the contraction of the bladder detrusor muscle?
Micturition involves the simultaneous coordinated contraction of the bladder detrusor muscle, which is controlled by parasympathetic (cholinergic) nerves, and the relaxation of the bladder neck and sphincter, which are controlled by sympathetic (α-adrenergic) nerves.
Where is micturition located in the spinal cord?
There is general consensus that a micturition “center” in the spinal cord is localized to segments S2-S4, with the major portion at S3.
Where are the somatic motor neurons located?
The somatic motor neurons that innervate the external urethral sphincter are located in the ventral horn (lamina IX) in Onuf's nucleus, have a similar arrangement of transverse dendrites and have an extensive system of longitudinal dendrites that travel within Onuf's nucleus1,37. Open in a separate window. Figure 3.
Where do parasympathetic preganglionic fibres travel?
Parasympathetic preganglionic fibres (shown in green) arise from the S2–S4 spinal segments and travel in sacral roots and pelvic nerves (PEL) to ganglia in the pelvic plexus (PP) and in the bladder wall. This is where the postganglionic nerves that supply parasympathetic innervation to the bladder arise.
What is the effect of NGF on sensory nerves?
NGF released from muscle or the urothelium can exert an acute and chronic influence on the excitability of sensory nerves through an action on tyrosine kinase A (TrkA) receptors . ATP released from efferent nerves or from the urothelium can regulate the excitability of adjacent nerves through purinergic P2X receptors.
Where does sympathetic innervation originate?
The sympathetic innervation arises in the thoracolumbar outflow of the spinal cord, whereas the parasympathetic and somatic innervation originates in the sacral segments of the spinal cord . Afferent axons from the lower urinary tract also travel in these nerves. Open in a separate window. Figure 1.
Does dopamine inhibit micturition?
For example, dopamine elicits inhibitory effects on micturition through D1-like receptors and facilitatory effects through D2-like receptors. On the other hand, activation of 5-HT1Areceptors inhibits micturition in the cat but facilitates it in the rat80. Neuroplasticity and pathology. Developmental changes.
Is sphincter relaxation present in infants?
In the infant (a) sphincter relaxation is present but less complete. On the other hand, in the paraplegic patient (c) the reciprocal relationship between bladder and sphincter is abolished. During bladder filling, involuntary bladder contractions (detrusor overactivity) occur in association with sphincter activity.
Does NGF cause afferent sensitization?
NGF has been implicated in the mechanism of afferent sensitization because cystitis, like SCI, increases NGF expression in the bladder111. Chronic administration of NGF to the bladder or the spinal cord mimics the effect of chemically induced cystitis on IAcurrents and firing in bladder afferent neurons112.
